Police said that the 15-year-old girl and her 16-year-old friend offered to collect milkshakes for the younger girl's parents from a local fast food restaurant.

Officers in Rocklin, California, allege that the drinks were spiked with prescription sleeping pills and that the parents drank about half of them before complaining that they tasted "grainy", according to the Sacramento Bee newspaper.

The parents are said to have woken the following morning feeling unwell and went to the local police station to ask for a drug testing kit. The parents later reported the incident to officers and the girl's were arrested.

Lon Milka, a spokesman for the Rocklin Police Department, said that the internet connection in the house was routinely cut off at 10pm.

He said: "The girls wanted to use the internet, and they'd go to whatever means they had to." He added: "They wanted to use the internet after 10pm and we believe, based on our understanding, that they felt the best method was to drug the parents." The teens were arrested on charges of conspiracy and wilfully mingling a pharmaceutical with food.